This study aimed to test acute reactivity during a physical activity in an outdoor setting and to verify the relative perceived performance. In all, 38 volunteers wore accelerometers or not and completed two 20-minute sessions of self-selected pace physical activity. Covered distance, exertional responses, and perceived efficacy were recorded at the end of every session. Relevant finding of this study has been that reactivity to accelerometers also occurs in acute condition. Consequently, this condition leads to a better performance and a greater perceived exertion. Moreover, this situation seems to occur in a state of awareness.